*- - 



!• 





ft 



AND THE SANDWICH ISLANDS 



129 



Uppes 

 Galleet 



volume, and so impregnated by them as to be rendered 

 cellular. These, from the intensity and suddenness of ex- 

 plosions and other causes, often become divided and tritu- Wall-easel, 

 rated into fine grains and powder, so light as to be borne great 

 distances by the winds, and sometimes darkening the air. 



(Wall 



) Under favourable 



a 



(Case 2>, Nos. 



64, 68 to 81) form Tufa, Peperino, &c, or falling into or 

 being subsequently carried into the sea, enclose and fossillize 

 shelly and other organic bodies (Case 2t } No. 66). 



H. W. B , 









: 



■ 







Shelf 3, containing specimens numbered from 20 to 42a, are 

 from the modern volcanos of the Sandwich Islands, 



Nos. 21 to 32, and No. 34, furnish good examples of the 

 cellular structure assumed by lava that has undergone perfect 

 fusion, and has been permeated by gases or aqueous vapours. 

 They also denote the manner in which streams of lava so 

 penetrated by gases have had their vesicles or air cham- 

 bers elongated and drawn out during the process of flowing. 

 So long as the lava retains sufficient heat for it to flow, and 

 to allow its particles to move freely amongst themselves, 

 the air vesicles undergo a change of form, and are elon- 

 gated or drawn out in the direction traversed by the moving 

 mass. Examples of this result of the flow of a heated 

 viscous mass are shown in Wall-case 1, Nos. 29, 30-36, 



and Wall-case 



, Nos. / and 8. 



The outer portions of the stream which have I ecu most 

 rapidly cooled assume a scoriaceoiis or slag-like aspect, 

 instances of which are afforded by 21 to 25, 27, 31, and 35. 



A variety of this scoriaceous lava, 35, is partially coated 

 with small crystals of specular iron, which have been depo- 

 sited on its surface by sublimation. 



Other specimens, 36 to 41, contain included crystals of 

 augite and olivine. The presence of these minerals is, pro- 

 bably, generally due to the separation of the substances of 



E 



